Since the requirements on the power density and efficiency of the power supply and the circuit efficiency are more and more critical, the resonant converters are becoming more and more popular due to their high efficiency.
However, the light-load efficiency of the resonant converters still cannot meet the requirements yet. It is because that the resonant converters need a magnetizing current having a specific value to realize a soft-switching while under the light load condition, this will cause certain constant losses such as the conduction loss and the switch loss, and these losses possess a higher ratio when they are under the light-load condition than they are under the heavy-load condition.
FIG. 1 is a circuit diagram of an LLC series resonant converter in the prior art. As shown in FIG. 1, an LLC series resonant full-bridge converter includes a resonant tank having three series-connected elements: a resonant inductor Lr, a resonant capacitor Cr and a magnetizing inductor Lm. The resonant converter receives an input voltage Vin, generates an output voltage Vo, and further includes an input capacitor Ci, switches Q1-Q4, a transformer T having a primary winding and two secondary windings, a rectification circuit having synchronous switches Q5-Q6, an output capacitor Co and an load RL. FIG. 2 shows operating waveforms of the LLC series resonant converter operating under a light-load condition in the prior art. In FIG. 2, VQ1-VQ4 are driving signals of the primary side switches Q1-Q4 respectively; VQ5-VQ6 are driving signals of the secondary side switches Q5-Q6 respectively; iLr is the waveform of the resonant current; iLm is the waveform of the magnetizing current; and the difference between iLr and iLm is the primary side current of the ideal transformer T, iPo. In the region A, energy transfers from the secondary side to the primary side, and in the region B, energy transfers from the primary side to the secondary side. At the mean time, to guarantee a fully soft-switching of the LLC resonant converter, the magnetizing current iLm must be kept at almost the same level during a light-load condition as that during a heavy-load condition. And all these increase the losses of the circuit. Thus, without any improvements, the light-load efficiency of the above-mentioned resonant circuit is much lower than the heavy-load efficiency of the same.
To increase the light-load efficiency of the resonant circuit, the burst mode control method is often adopted. FIG. 3 shows the operating waveforms of the LLC series resonant converter under a light-load condition wherein the resonant converter adopts the burst mode control method. In FIG. 3, Vds is the voltage across the drain and the source of the switch Q2/Q3; t0-t4 is a burst period comprising several operating period; t0-t2 is the working time and t2-t4 is the breaking time. Under this kind of operating method, the output capacitor Co is charged only during the working time t0-t2. While during t2-t4, the resonant converter stops operation, and the output capacitor Co is discharged and provides energy to the load RL. During the working time t0-t2, the resonant converter operates just like it is under the heavy-load condition. Thus the light-load efficiency of the resonant converter adopting the burst control method is almost equal to the heavy-load efficiency during the work time t0-t2.
But under this burst mode control method, the driving of the synchronous rectifiers in the secondary side is generally not optimized and this will influence the circuit's efficiency. There are mainly two ways to deal with the driving of the synchronous rectifiers under the burst mode control. One is to turn on the synchronous rectifiers just at the same timing that the corresponding primary switches are turned on (ignoring the timing difference between the turning on signals of the primary switches and the secondary side synchronous rectifiers due to the propagation delay). As shown in FIG. 3, VQ5 (VQ6) is the driving signal of the synchronous rectifier Q5 (Q6); VQ1, VQ2, VQ3 and VQ4 are the corresponding driving signal of the primary switches Q1, Q2, Q3 and Q4. Switch Q5 is turned on at the same timing as that of Q1 and Q4; switch Q6 is turned on at the same timing as that of Q2 and Q3. Before the timing t0, the resonant tank of the resonant circuit has experienced the oscillations; the currents of the resonant inductor and the magnetizing inductor and the voltage across the resonant capacitor are close to zero basically; and after the synchronous rectifier is turned on at the timing t0, the voltage of the magnetizing inductor Lm of the primary side equals to the voltage across the output capacitor Co and the energy is transferred back from Co to the primary side. As shown in FIG. 3, the output voltage Vo appears a dramatically drop in t0-t1 due to the energy's transferring back, and this results in a large iLr in the next operating period and the increasing power loss. The other method is that the synchronous rectifier does not operate when the circuit operates under the burst mode control method. That is to say, the secondary side current flows through the body diodes of Q5 and Q6 to charge the output capacitor Co during the working time period t0-t2 of the burst period, and this prevents the energy's transferring back from the secondary side to the primary side. But the drawback is the conduction loss is increased since the forward voltage drop of the body diode is much larger than that of the synchronous rectifier.
To resolve the aforementioned conventional controlling problems, a controlling method is proposed in the present invention so as to raise the light-load efficiency of the resonant circuit to its extreme limit.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a synchronous rectification circuit having a burst mode controller and a controlling method thereof.